With the recent popularization of photographic equipment such as cameras, there have been increased opportunities of taking pictures using silver halide photographic light-sensitive materials.
There has also been increasing demand for higher sensitivity and higher image quality.
The silver halide grain is a factor affecting the improvement in sensitivity and image quality of silver halide photographic light-sensitive materials. Traditionally, there have been attempts to develop silver halide grains offering improved sensitivity and image quality.
However, silver halide grain size reduction for image quality improvement, a common practice, tends to cause sensitivity reduction, posing a limitation on meeting the requirements of both high sensitivity and high image quality.
With the aim of achieving further improvement in sensitivity and image quality, there have been investigations of improving the sensitivity/size ratio per silver halide grain, including the use of tabular silver halide grains as described in Japanese Patent Publication Open to Public inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 111935/1983, 111936/1983, 111937/1983, 113927/1983 and 99433/1984.
In comparison with so-called normal crystal silver halide grains such as octahedral, decahedral or hexahedral grains, these tabular silver halide grains have a larger surface area per unit volume of silver halide grains, thus permitting adsorption of a larger amount of sensitizing dyes to the silver halide grain surface, which in turn offers an advantage of further sensitivity improvement.
Japanese Patent O.P.I. Publication No. 92942/1988 discloses an art in which a core of high silver iodide content is formed in tabular silver halide grains. Japanese Patent O.P.I. Publication No. 151618/1988 discloses an art using hexagonal tabular silver halide grains. These publications describe effects on sensitivity and graininess, respectively.
Also, Japanese Patent O.P.I. Publication Nos. 106746/1988 and 279237/1989 describe arts using tabular silver halide grains having a substantially layered structure parallel to the two mutually opposite major planes, or tabular silver halide grains having a layer structure divided by a plane substantially parallel to the two mutually opposite major planes wherein the average silver iodide content of the outermost layer is higher by at least 1 mol % than that of the whole silver halide grain, respectively.
Japanese Patent O.P.I. Publication No. 183644/1989 discloses an art using tabular silver halide grains characterized by a perfectly uniform silver iodide distribution in the silver halide composition containing silver iodide.
There are also some reports of arts concerning mutually parallel twin planes in tabular silver halide grains. For example, Japanese Patent O.P.I. Publication No. 163451/1988 discloses an art using tabular silver halide grains wherein the ratio (b/a) of grain thickness (b) to the longest distance (a) between two or more mutually parallel twin planes is not lower than 5. Japanese Patent O.P.I. Publication No. 201649/1989 discloses an art wherein the number of transition lines is also specified. These publications report on effects on sensitivity, graininess and sharpness.
WO91/18320 reports on an art using tabular silver halide grains wherein the distance between at least two twin planes is less than 0.012 microns. Japanese Patent Application No. 353043/1991 reports on an art using core/shell twin crystal silver halide grains wherein the average maximum twin plane distance is 10 to 100 .ANG.. These publications describe improving effects on sensitivity and graininess, or sharpness, pressure properties and graininess, respectively.
In view of the present state of commercial distribution and consumption of silver halide photographic light-sensitive materials, it is very important to stably offer high sensitivity and high image quality at the time of silver halide photographic light-sensitive material purchase and actual picture taking by the user, i.e., after long-term storage on the market, rather than just after production, or at the time of shipment, of the silver halide photographic light-sensitive material.
Usually, this market storage time is about several months to 2 years. The prior art using conventional tabular silver halide grains is unsatisfactory in meeting the requirements for high sensitivity and high image quality as evaluated after long-term storage or in a severe forced deterioration test, though it is common practice to evaluate the photographic performance of the light-sensitive material after relatively short-term storage or in a mild forced deterioration test. In this situation, there has been demand for the development of a better art.
With this in mind, the present inventors investigated the above problems and found that it is effective in improving the sensitivity and graininess of tabular silver halide grains after long-term storage to define the average distance between two or more twin planes parallel to the major plane within an appropriate range and concurrently monodisperse the silver halide grains.
The present inventors also made investigations based on the assumption that photographic performance stability deterioration, particularly photographic performance deterioration after long-term storage, in a silver halide photographic light-sensitive material produced with a silver halide photographic emulsion containing conventional tabular silver halide grains, is associated with grain-to-grain dispersion of grain size distribution, and in the case of silver iodobromide, associated with grain-to-grain dispersion of silver iodide content, i.e., unevenness among the silver halide grains, and found that this unevenness is caused by too narrow a distance between two or more twin planes parallel to the major plane of the tabular grain.
Another finding was that when the distance between two or more twin planes parallel to the major plane is too wide, the tabular silver halide grains formed have an excessively low aspect ratio, which in turn hampers the improvement in sensitivity and image quality as a result of increase in the sensitivity/size ratio per silver halide grain, an aspect ratio which has been thought as a benefit from the use of tabular silver halide grains.